This invention relates generally to solar cells, and, more particularly, to a solar cell array having an improved ratio of power output to weight.
Semiconductor solar cells are utilized to convert light energy to useable electrical voltages and currents through the photovoltaic effect. Briefly, a typical semiconductor solar cell includes an interface between n-type and p-type transparent semiconductor materials. Light shining on the semiconductor materials adjacent the interface creates hole-electron pairs in addition to those otherwise present, and the minority charge carriers migrate across the interface in opposite directions. There is no compensating flow of majority carriers, so that a net electrical charge results. A useful electrical current is then obtained in an external electrical circuit by forming ohmic contacts to the materials on either side of the interface.
In general terms, a photovoltaic solar cell is fabricated by depositing the appropriate semiconductor layers onto a substrate, and then adding additional components to complete the cell. The most common type of solar cell is the n-on-p silicon solar cell, wherein a layer of n-doped silicon overlies a layer of p-doped silicon, so that the n-doped siliocon faces the sun. Gallium arsenide solar cells are of increasing interest, since such cells can produce 25 percent to 40 percent more power per unit area than a silicon solar cell. Gallium arsenide is, however, over twice as dense as silicon, so that the power output per unit weight of solar cell is less for a conventional gallium arsenide cell than a silicon cell.
The individual solar cells are connected together into large arrays to deliver power of the desired voltage and current. The ratio of power output to weight of the solar cell array is an important spacecraft design parameter, since the required power output could in principle be satisfied by larger numbers of low density, low output solar cells made of silicon, or by smaller numbers of high density, high output solar cells made of gallium arsenide. Large numbers of solar cells require more supporting structure, which adds weight and complexity to the spacecraft. Gallium arsenide solar cells continue to receive much attention, as methods are explored to overcome their weight disadvantage arising from the weight of the solar cell itself, so that advantage can be taken of the reduced weight of supporting structure required of such cells.
As an example of the fabrication of a solar cell, a p-on-n gallium arsenide solar cell is fabricated by epitaxially depositing a layer of n-type gallium arsenide onto a single crystal gallium arsenide substrate, and depositing a layer of p-type gallium arsenide overlying the layer of n-type gallium arsenide, so that the layer of p-type gallium arsenide faces the sun during operation. The interface between the p-type gallium arsenide and the n-type gallium arsenide forms the basic solar cell active region. External ohmic electrical contacts to the n-type and p-type layers are applied, and a voltage is measured across the contacts when light energy is directed against the interface. Optionally, a P+ layer of aluminum gallium arsenide may be deposited over the layer of p-type gallium arsenide to limit recombination of charge carriers. To protect the solar cell from physical contact and radiation damage such as encountered in a space environment, it is conventional to apply a transparent cover of glass over the solar cell components.
A number of the individual solar cells are connected together in an array, typically by fastening the solar cells to a support structure and then electrically interconnecting the cells into series and parallel arrangements, as necessary to meet the spacecraft power requirements. Presently operating earth satellites such as a Hughes Aircraft Co. HS-376 communications satellite may have as many as 20,000 silicon solar cells, each about 2 centimeters by 4 centimeters in size. The solar cells are typically arrayed either on a cylindrical structure which both supports the solar cells and also forms the exterior wall of the spacecraft, or on a wing-like structure extending outwardly from the body of the spacecraft. Since the cost of raising weight to orbit is high, the weight of the solar cells, their associated hardware, and the solar cell arrays is desirably reduced as much as possible. This incentive for improved power output and weight reduction is particularly pressing for solar cells such as gallium arsenide solar cells, which have higher power output per unit area than silicon solar cells, but continue to be at a disadvantage in power output per unit weight, because of their higher densities.
Thus, there is a continuing need for an approach for increasing the ratio of power output to weight for solar cells and solar cell arrays, particularly for those types of solar cells that are made of dense materials. An answer to this need should be compatible with existing technology and manufacturing operations for the solar cells and arrays, and should not be incompatible with further advances in these fields. The present invention fulfills this need, and further provides related advantages.